1. Technical Field of the Invention
The present invention relates generally to a signal processing device for use in a physical force measuring system which is designed to measure an applied physical force such as pressure or vibration using a piezoelectric device which may be employed in combustion pressure measuring device for measuring the combustion pressure of an internal combustion engine.
2. Background Art
Typical combustion pressure measuring devices for internal combustion engines use a charge amplifier designed as a signal processing circuit which handles an output of a combustion pressure sensor. The charge amplifier works to monitor a current outputted by a combustion pressure sensor in response to an applied pressure and integrate it to produce a voltage signal indicative of the combustion pressure. An example of a combustion pressure measuring device of this type is shown in FIG. 11.
A combustion pressure sensor 61 is coupled to an integrating circuit 62 made by a charge amplifier. An output of the integrating circuit 62 is transmitted to an external device through a high-pass filter 63, an amplifier 64, and a low-pass filter 65. The integrating circuit 62 is made up of an operational amplifier 66, a resistor 67, and a capacitor 68 and designed to integrate a charge produced in the combustion pressure sensor 61 to provide a voltage signal.
Specifically, the integrating circuit 62 integrates a weak current signal. Therefore, in order to prevent the capacitor 68 from being charged by a bias current from the operational amplifier 66, the operational amplifier 66 having a high input impedance is used to store the charges in the capacitor 68. The time constant of the integration depends upon the capacitor 68 and the resistor 67. The cut-off frequency of the integrating circuit 62 is relatively low, so that the resistance value of the resistor 67 will be several M xcexa9 to several tens M xcexa9. For instance, when the cut-off frequency of the integrating circuit 62 is 0.8 Hz, and the capacitance of the capacitor 68 is 0.1 xcexcF, the resistance value of the resistor 67 will be approximately 2M xcexa9.
The resistance value of the resistor 67 may be determined, as discussed below, in terms of a combustion cycle of the engine. The combustion cycle f of the engine using an engine speed during idling is
f=750 rpm/(60 sec.xc3x972 cycles)=6.25 Hz 
Considering the fact that ensuring the integrating operation requires the cut-off frequency of the integrating circuit 62 to be one-tenth ({fraction (1/10)}) of a frequency of a detected signal (i.e., the combustion cycle f), the resistance value Rin of the resistor 67 may be expressed by
Rinxe2x89xa71/(2xcfx80xc3x970.1xc3x97fxc3x97Cin) 
where Cin is 0.1 xcexcF. The resistance value Rin of the resistor 67 is, thus, 2.5M xcexa9.
The input impedance of the integrating circuit 62 is equal to that of the resistor 67 and thus has a higher value. The integrating circuit 62 is, therefore, undermined easily by the leakage current from the combustion pressure sensor 61 or an external device due to the humidity. This problem will be described in detail below with reference to time charts in FIGS. 12(a) to 12(d).
Usually, the combustion pressure of the engine, as shown in FIG. 12(a), rises every combustion in each cylinder. In the absence of the leakage current, the output of the integrating circuit 62 changes, as shown in FIG. 12(b), within a voltage range (e.g., 0 to 12V) defined near the voltage of a power supply of the operational amplifier 66. Specifically, the output of the integrating circuit 62 changes following each change in level of the combustion pressure. Since the output of the combustion pressure sensor 61 is inputted to an inverting input (xe2x88x92) of the operational amplifier 66, the output of the integrating circuit 62 is reverse in level to the input thereinto. The operational amplifier 66 needs to be actuated on xc2x112V. Specifically, a negative (xe2x88x92) power supply voltage must be prepared, therefore, this system is unsuitable for automotive vehicles.
In the presence of the leakage current, the output of the integrating circuit 62 is, as shown in FIG. 12(c), shifted in level to a negative side. This will result in an unwanted change in output of the combustion pressure measuring device even though a dc component is removed by the high-pass filter 63.
If the leakage current increases greatly, it will cause the waveform of the output of the integrating circuit 62, as shown in FIG. 12(d), to be deformed greatly, so that the output indicative of the combustion pressure will be saturated at a minimum output voltage VL of the operational amplifier 66 (i.e., the voltage of the negative power supplied to the operational amplifier 66). Specifically, a great leakage current causes an error in the output of the combustion pressure measuring device to be produced.
Further, if electric noises arise in a driver for fuel injectors of the engine, it will cause the output of the integrating circuit 62 to change, thus resulting in decreased accuracy of measuring the combustion pressure.
Japanese Patent First Publication No. 4-97614 teaches a signal processing circuit for a touch sensor equipped with an piezoelectric element. The signal processing circuit consists of a differentiating circuit, a diode, an integrating circuit, and a comparator. The differentiating circuit receives the electromotive force produced by the piezoelectric element. The diode rectifies an output of the differentiating circuit. The integrating circuit receives an output of the diode. The comparator compares an output of the integrating circuit with a reference voltage. Specifically, the differentiating circuit extracts only a change in electromotive force produced by the piezoelectric element. The integrating circuit stabilizes of the change in electromotive force for increasing the reliability of the output of the sensor.
The above system is designed to determine whether a cumulative value of loads applied to the piezoelectric element is greater than a reference level or not, meaning that the output of the diode only shows an increase in load applied to the piezoelectric element and thus unsuitable for processing a sensor signal indicating the pressure containing a vibrational component such as the combustion pressure of the engine.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a signal processing device capable of processing an output of a piezoelectric element without errors caused by the leakage current and/or electric noises.
According to one aspect of the invention, there is provided a signal processing device which processes an output of a piezoelectric sensor to produce a signal indicative of a load applied to the piezoelectric sensor. The signal processing device comprises: (a) a differentiating circuit differentiating the output of the piezoelectric sensor; and (b) an integrating circuit integrating an output of the differentiating circuit to produce an output signal as a function of the output of the piezoelectric sensor. A frequency range of the output signal is determined based on a frequency range of the load applied to the piezoelectric sensor.
In the preferred mode of the invention, a cut-off frequency of the differentiating circuit is determined based on an upper limit of the frequency range of the load applied to the piezoelectric sensor. A cut-off frequency of the integrating circuit is determined based on a lower limit of the frequency range of the load applied to the piezoelectric sensor.
The piezoelectric sensor includes a piezoelectric element which provides the output changing with a change in physical pressure applied to the piezoelectric element.
The piezoelectric sensor may be installed in a combustion engine to produce the output in response to a change in combustion pressure in a cylinder of the combustion engine. The differentiating circuit extracts from the output of the piezoelectric sensor a component indicative of the change in combustion pressure. The cut-off frequencies of the differentiating circuit and the integrating circuit are determined based on an available range of speed of the combustion engine.
The differentiating circuit may be made up of a capacitor component of the piezoelectric sensor and a resistor.
An operational amplifier may also be provided in which the output of the piezoelectric sensor is inputted to a first input terminal of the operational amplifier, and a positive reference voltage is inputted to a second input terminal of the operational amplifier.